Power amplifiers typically are assigned peak power level ratings. One risk is that an amplifier, i.e., normally the main transistor, breaks down if it is exposed to a high peak level. A second risk is that the amplifier itself has a non-linear behavior which may limit high output portions of the signal and include increased emission levels at frequencies outside an intended or allowed spectrum. Reducing power may potentially minimize these particular issues, but will create reduced efficiency levels for the radio equipment. Another example solution includes crest factor reduction (CFR) which can he implemented in a transmitter to reduce peak power in relation to the average power. This can be achieved by directly reducing the peak power by a baseband rearrangement of the baseband signal or by forcing the signal down at peak levels by clipping.
Clipping can be performed in a baseband signal configuration or on a combined signal. Clipping is a form of distortion that limits a signal once it exceeds a threshold. It may be described as hard, in cases where the signal is strictly limited at the threshold, producing a fiat cutoff which results in many high frequency harmonics and intermodulation distortion components.
In a digital system, the sampling rate for complex signals must be as high as the instantaneous signal bandwidth (IBW) is wide according to the well known Nyquist sampling theorem. Instantaneous signal bandwidth (IBW) is defined as the total bandwidth encompassing all the carriers intended for transmission. Single carrier signals, having a smaller channel bandwidth (CBW), e.g. 5 MHz or 20 MHz, do not require significant signal processing speeds. However, when a signal to be transmitted includes multiple carrier bands separated by a significant frequency bandwidth, for example 20-100 times the channel bandwidth (CBW), instantaneous signal bandwidth quickly becomes a detrimental factor in signal processing speeds.
Widely-separated carriers, i.e., having a carrier center frequency separation of much greater than twice the channel bandwidth of the carrier bands (>>2CBW), require processing speeds which are not practical for existing hardware. As an example, a widely-separated carrier signal including simultaneous transmission into 3GPP Band 1 (2110-2170 MHz) and Band 7 (2620-2690) means a maximum frequency separation of 580 MHz between the bands if an LTE carrier channel bandwidth of 20 MHz is used. Using this example, a matching sampling rate and processing speed of at least 580 MHz would be required, plus an additional spectrum margin. All together, the processing speed would exceed most hardware capabilities currently available.
Hard clipping of a signal to be transmitted also produces unwanted emissions outside the intended spectrum. Such emissions usually do not comply with requirements set up by standardization bodies. Therefore, filtering is often applied when hard clipping is used to introduce crest factor reduction (CFR) methods. Other CFR methods are also available which do not require hard clipping. For example, an article of M. R. Schroeder, “Synthesis of low-peak-factor signals and binary sequences with low autocorrelation, “IEEE Transactions on Information. Theory, vol. IT-16, pp. 85-89, January 1970, teaches a phase adjustment of each continuous-wave tone (CW-tone) used to decrease the peak-to-average ratio (PAR) of the signal without hard clipping it.
Clipping can be viewed as if a certain spectrum is added to the original spectrum of isolated carriers just covering the instantaneous bandwidth (IBW) of the carriers. The terms “adding a clipping spectrum” or “adding crest factor reduction signal components” will be used throughout the specification and drawings even though the actual signal amplitudes are limited or decreased by the clipping operation. The tolerated spectrum inside the desired transmission band is usually larger than the unwanted spectrum outside the carriers. Some examples of this are found in the telecom standards defined by 3GPP. Unwanted emissions are defined via specified allowed levels in certain frequency ranges, by adjacent channel leakage power ratio (ACLR, and via a spectrum mask, both defined just outside the channel bandwidth of the carriers. Inside the carriers, the unwanted emission requirement is often converted into an error vector magnitude (EVM) that must be below a certain limit.
CFR methods essentially address the question of how to compute the necessary extra spectrum that brings the PAR down to a predefined level but at the same time fulfill the requirement of achieving a low EVM figure and low unwanted emissions outside the carriers. Illustrative methods include, but are not limited to: 1) a method developed by Ericsson as described in R. Hellberg, “Apparatuses and a method for reducing peak power in telecommunications systems”, PCT/SE2006/050237, hereby incorporated by reference and referred to hereafter as “Ericsson clipping algorithm”, 2) peak windowing as described in an article by Mistry, Hiten N., “Implementation of a peak windowing algorithm for crest factor reduction in WCDMA”, Master Of Engineering Thesis, Simon Fraser University, 2006, and 3) tone reservation as described in a thesis to J. Tellado, “Peak to Average Power Reduction for Multicarrier Modulation”, Ph.D. thesis, Stanford University, 2000. Additional example methods are summarized in an article to V. Vijayarangan, R, Sukanesh, “An overview of techniques for reducing peak to average power ratio and its selection criteria for orthogonal frequency division multiplexing radio systems”, Journal of Theoretical and Applied information Technology, Vol 5, No 5., pp 25-36, 2009.
All these methods try to satisfy common design goals for EVM and unwanted emissions. The technology described below can be used along with any of these methods, or others, both known and future, to provide crest factor reduction for multi-band signals.
A common outcome of existing solutions is that combined carriers before and after clipping are the same but also contain some clip distortion. This clip distortion is normally contained inside the carriers, hiding the unwanted spectrum emissions to the carriers. The amount of clip distortion, in relation to the carrier power itself, defines the error vector magnitude (EVM) of the signal. Existing telecommunication standards set a maximum EVM threshold to ensure satisfactory demodulation at the receiver end. This maximum EVM threshold may vary with bit rate and transmission configuration, e.g., supporting MIMO.
The technology described herein addresses the problem of clipping carriers in a multi-carrier system. As previously described, existing approaches may require a very high sampling speed in order for the method to work over the entire multi-carrier or combined signal. Clipping has to be made on the combined signal to be effective. Separate clipping on individual ones of the multiple carriers at a low speed requires a non-optimal testing step and introduces high signal latency and increased complexity.
What is needed is a solution that reduces sampling rates required to clip carriers in a multi-carrier system where the sampling speed would be too high for a practical solution to work.